"Defense" means protecting your flag carrier against enemy attacks. It also means defending your base from enemies who are trying to get your flag.
Holding the Flag
There are lots of tricks and tactics. Here are a few key items.
They can see you. After awhile, the map shows where the flags are. So hiding out is not as important as running and cc.
If possible, stay near the flag well. If your side drops the efc, you want to be able to cap fast.
Be ready to heal yourself. Your healer may suddenly be stunned, feared or sheeped. Have a healing potion where you can get to it quickly.
Try not to fight. If you have offensive abilites, use them sparingly during a fight. The best time is when an enemy is near death and you can knock them out. Mostly you want to stay as healthy as you can.